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International Journal of Civil Engineering and Technology (IJCIET)
Volume 9, Issue 10, October 2018, pp. 201–212, Article ID: IJCIET_09_10_021
Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=10
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
©IAEME Publication Scopus Indexed
THE INFLUENCE OF CRUMB RUBBER
MODIFIER (CRM) ON THE PROPERTIES OF
ASPHALT CONCRETE MIXTURES
Esraa T. Al-Azawee
Assistant Lecturer, Civil Engineering Department,
AL-Mansour University College, Baghdad, Iraq
Zaynab I. Qasim
Assist. Professor, Civil Engineering Department,
University of Technology, Baghdad, Iraq
ABSTRACT
The use of rubber crumb as additives has been reported to improve the service life
of pavements, require less maintenance, and drive comfort. The aim of this paper is to
summarize the results of the effect of CRM on the basic properties of hot asphalt
mixtures with different CRM percentages. During the study, four different percentages
of CRM were used. The experimental work consisted of the preparation of rubberized
asphalt by blending asphalt cement with crumb rubber (0, 5, 10, and 15 % by asphalt
weight) using the wet method. Next is the preparation of Marshall samples to
determine the optimum asphalt percentage using the Marshall method. The laboratory
tests include Marshall stability and flow, and ultrasonic test. The results from this
study showed that the use of rubberized asphalt binder in mixes increased the
optimum asphalt content and enhanced the volumetric properties of asphalt mixtures
in terms of the Marshall stability and Marshall flow. However, the use of 10 % CRM
showed the best effect on the Marshall stability of the mixture. The experimental
outcome showed that the incorporation of CRM as additives in mixtures significantly
influenced the evaluated properties of the mixtures.
Key words: Asphalt mixture; Crumb rubber modifier (CRM); Marshall stability;
ultrasonic test.
Cite this Article: Esraa T. Al-Azawee and Zaynab I. Qasim, The Influence of Crumb
Rubber Modifier (CRM) on the Properties of Asphalt Concrete Mixtures,
International Journal of Civil Engineering and Technology (IJCIET) 9(10), 2018, pp.
201–212.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=10
1. INTRODUCTION
Innovative and environmentally friendly ideas on how to re-use industrial and domestic waste
products have been developed due to the increasing environmental concerns associated with
Esraa T. Al-Azawee and Zaynab I. Qasim
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their presence in the environment. Their increased presence in the environment is a serious
source of concern due to their associated pollution tendencies. They have been used as cost-
effective and environmentally friendly materials in modification processes to reduce damages
to pavements. The rapid increase in the total number of automobiles globally has contributed
greatly to the increase in the generation of waste tires which has become a major
environmental problem. The incorporation of rubber crumbs derived from waste tires into
asphalt mixtures have been considered as a sustainable construction approach and has been
widely investigated as enhancers of modified asphalt mixtures properties [1-6]. As stated by
many researchers, the physical properties of the control asphalt can be enhanced by using
different industrial waste additives such as fly ash and phospho-gypsum [13,14].Rubber
crumbs incorporation into bitumen binder has been reported to improve the physical
properties of the modified binder as evidenced by the reduced penetration tendency and
binder durability [1]. Rubber crumbs have been shown through various experiments to
improve the physical characteristics of modified asphalt mixtures [2-4].
2. BACKGROUND AND SIGNIFICANCE OF WORK
Annually, more than 1.4 billion tires are sold worldwide, and this number of tires will
eventually become waste tires or all within the category of End of life tires (ELTs) with time
[7]. In Europe, the number of ELTs is bound to increase as the number of vehicles in that
region continues to increase. The large volume and durability of these tires make them more
problematic to manage. Premature pavement failures and accumulation of ELTs are both
interconnected and dependent of each other due to the high axle loading and the enormous
increase in traffic density, respectively. Rubber crumbs were first used as asphalt pavements
additives in the past 170 years ago, with the first experiment that involved the addition of
natural rubber into bitumen being reported in the 1840s [8]. These studies attempted to
demonstrate the flexibility of rubber in the pavement industry. Nowadays, different quality-
related problems are addressed with the use of rubberized bitumen materials which are
produced through wet processes. The effectiveness of this strategy is demonstrated by the
stability of roads that were constructed in the last three decades [9]. Rubber crumbs can be
introduced into asphalt mixtures through either wet or dry processes [5]. The wet process
involves the addition of rubber crumbs into hot asphalt and allowing the asphalt to react with
the rubber. During this process, the major event is the swelling of the rubber. However, in the
dry process, the rubber crumbs are first mixed with a hot aggregate before being added to the
bitumen. The resistance of an asphalt mixture to high temperate deformation and low-
temperature cracking can be improved through the addition of rubber crumbs via a dry
process [6]. Only the specified volumetric properties of an asphalt mixture can be obtained
through the wet process [3]. This work mainly aims at investigating the influence of rubber
crumbs addition into asphalt mixtures via the wet process.
3. RESEARCH METHODOLOGY
The testing program consists of physical tests that include penetration, specific gravity,
ductility, and softening point for asphalt binder. The mechanical tests include the Marshall
test and the ultrasonic test. The testing phase consisted of the preparing the asphalt mixture
samples, as well as the laboratory investigations before and after adding CRM to the mixtures.
3.1. Material
Locally available materials were utilized in this research, including the asphalt binder, mineral
filler, crushed aggregate, and CRM additive. This work employed an asphalt cement grade of
40-50 which was obtained from Al-Doura Refinery located in the south-west of Baghdad. The
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coarse and fine aggregates (Figure 1) used were sourced from AL- Nibaie quarry. The filler
used is a non- plastic material passing sieve No.200 (0.075mm). The filler is usually used to
fill the voids and enhance the property of the mixtures. Limestone dust was used as a mineral
filler during the study; it was sourced from the lime factory of Karbala’ governorate (Figure
2-a). The rubber crumbs used as an additive in this study were sourced from tire factories in
AL-Najaf governorate Figure 2-b. the rubber crumbs are black granules which were produced
from recycled tires. They have a specific gravity of 1.13. There are several types of
compounds in tires and the component that has the most effect on the physical properties of
modified asphalt rubber (AR) is the hydrocarbon content of the rubber; however, additional
effects can also come from the natural rubber content [10]. The rubber crumbs were sieved to
the desired sizes by passing the shredded material through No. 8 (2.36 mm) and No.50 (0. 3
mm) sieves.
Figure 1 Course and fine aggregate used in study
Figure 2 (a): Limestone mineral filler, (b) Crumb rubber used in this Study
3.2. Preparation of Modified Asphalt Cement
A wet process was followed during the preparation of the modified asphalt in this study. At
first, the asphalt cement subjected to heating (at 150 ˚C) before being mixed with different
percentages (5, 10, and 15 % by asphalt weight) of rubber crumbs. This blending was done
using a laboratory mixer (Figure 3) at a blending speed of 1300 rpm and at 170 °C.
Figure 3 Blending apparatus for mixing the rubberized asphalt
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3.3. Preparation of Asphalt Mixtures
The Marshall mix design method was deployed for both the unmodified and rubber crumb
modified asphalt mixtures in this study. This Marshal mix design method is commonly used
in Iraq to design asphalt mixtures. This study involved several laboratory examination stages;
the first stage involved the selection of the aggregates, comprising of determining their
physical properties and composite grades that will meet the requirements for asphalt mixtures.
This stage followed the specification provided by the General Standards of the SCRB (R/9,
2003) [11]. The second stage involved the evaluation of the asphalt for both unmodified and
modified asphalt mixtures; the optimum asphalt content for each asphalt mixture was also
determined in this stage. The final stage is the verification of the volumetric parameters. At
this stage, three samples were prepared for each mixture. The rubber crumbs were added to
the asphalt mixtures using a wet process. The rubber crumbs were added at different
percentages of 5, 10, and 15 % by asphalt mixture weight. The asphalt was heated to about
150 °C prior to the addition of the rubber crumbs. This was aimed at the production of a
kinematic viscosity of 170 ± 20 centistokes. The temperature of the mixture was maintained at
the range of I35 – 150 °C. The mixture was manually mixed until homogeneity was achieved.
Having homogenized, the asphalt cement was added to the heated aggregate at the desired
amount and thoroughly mixed manually for about 2 minutes until the aggregates were fully
coated with asphalt.
4. RESULTS AND DISCUSSION
4.1. Aggregates
Table 1 showed the results of the tests on the general properties of the prepared aggregate in
this study while Table 2 and Figure 4 showed the result of the sieve analysis of the tested
aggregates and Figure 4. The combined aggregate gradation for the asphalt mixture is also
shown in Table 2. These results were determined following the required specifications and
compared with the specifications of SCRB (R/9, 2003) [11].
Table 1 Physical characteristics of the fine and Coarse Aggregates
Laboratory Test ASTM Designation Test Results
Coarse Aggregate
BSG C-127 2.62
ASG C-127 2.68
WA % C-127 0.46
Fine Aggregate
BSG C-128 2.64
ASG C-128 2.71
WA % C-128 0.72
Note: BSG = Bulk specific gravity, WA = Water absorption, ASG = Bulk specific gravity
Table 2 Selected aggregate gradation
Sieve
Size 3/4" 1/2" 3/8" No.4 No.8 No.50 No.200
%
Passing 100 95 83 58 36 12 5
SCRB 100 90-100 76-90 44-74 28-58 5-21 4-10
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Figure 4 Selected aggregate gradation and specification limits
4.2. Asphalt Binder
Tables 3 and 4 showed the properties of modified and unmodified asphalt. The results showed
that a higher rubber crumbs percentage strongly affected (reduced) the penetration and
ductility of the resulting asphalt while lower percentages increased the softening point of the
asphalt. All the observed physical properties of the produced asphalts were within standard
requirements. The higher softening point and lower penetration of the modified asphalt
showed that the additive caused an increased stiffness of the resulting asphalts, but with
reduced flexibility. The lower ductility showed poor adhesive properties of the asphalts. The
decreased ductility can be due to the fact that the modified asphalt was manually blended, and
this could have an effect on the bitumen-crumb rubber interaction. Note that the blending
phase is a critical step towards ensuring mixture homogeneity which defines the
characteristics of the asphalt mixture.
Table 3 Physical properties of asphalt cement 40/50 penetration
Asphalt Property Units ASTM
designation
Test Results SCRB
specification
Penetration at 25 ˚C,
100 gm, 5sec
0.1 mm D5 46 4050
Flash Point, ˚C D92 256 >232
Ductility at 25 ˚C,
5cm/min
cm. D113 136 >100
Softening Point ˚C D36 52 -
Specific gravity - D70 1.03 ˃1.0
Table 4 Properties of crumb rubber modified asphalt
Asphalt Property Units ASTM
Designation
Test Results Standard Req.
(modified
asphalt) Crumb rubber
5% 10% 15%
Penetration at 25 ˚C,
100 gm, 5sec
0.1
mm
D5 40 32 27 Min.20
Flash Point, ˚C D92 265 270 277 >232
Ductility at 25 ˚C,
5cm/min
cm D113 125 97 65 >100
Softening Point ˚C D36 57 62 70 -
Specific gravity - D70 1.03 1.03 1.03 ˃1.0
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The results showed that the degree of penetration of the modified bitumen was
significantly decreased with the addition of CRM. The addition of CRM increased the
viscosity of the bitumen because of the increased rubber mass. The penetration degree was
also decreased by 13, 30, and 40 % with the addition of 5, 10, and 15 % rubber crumbs,
respectively (Figure 5-a). The softening point was noted to increase with an increasing CRM
percentage due to the increased hardening of the bitumen (increased by 10, 19, 35 % with an
increasing CRM percentage of 5, 10, 15 % respectively). This can be related to the differences
in the rubber viscosity from that of bitumen (Figure 5-b). Similarly, the ductility value of the
modified bitumen was decreased compared to that of the control. This was due to the oil ratio
in the bitumen that was absorbed by the rubber particles which decreased gradually with
increased CRM percentage. The observed percentage decrease was 8, 29, and 52 % with the
addition of 5, 10, and 15 % CRM, respectively.
Figure 5 Physical properties of CRM
4.3. Mineral Filler
A summary of the physical properties of the limestone dust used in this study is presented in
Table 5. Table 5 Summary of limestones’ physical properties
4.4 Crumb Rubber
The materials specification and physical properties of CRM based on the recommendation
of Company for Tire Industry in AL-Najef City - Engineering Office -Technology
Department are presented in Table 6.
Table 6 CRMs’ physical properties and material specifications
Properties Test results
Percentage passing No.200 (0.075 mm ) 96 %
Plasticity index N.P.
Specific gravity 2.71
Property or Characteristic Unit Requirement or value
Specific Gravity - 1.13
Density grn/m3 1.320
Young's Modulus (E) MPa 2600 - 2900
Tensile Strength MPa 40 - 70
Elongation at Break % 25 - 50
Melting Point % 200
Rubber Hydrocarbon % 48 min
Carbon Black % 25 - 35
Acetone Extract % 10 - 20
Ash at 550 % 8.0 min
Metal Content % 0.03 max
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5. RESULTS AND DATA ANALYSES
5.1. Optimum Asphalt Content
The optimum asphalt percentage of the unmodified and modified mixtures was determined
using a Marshall test method. The test was conducted using five binder percentages (4, 4.5, 5,
5.5 and 6 %) and three samples for each binder percentage. The sample preparation procedure
is shown in Figure 6 while the relationship between the asphalt content and the Marshall
properties of the control mixture is shown in Figure 7. The optimum asphalt content for the
selected gradation and control asphalt binder was 4.8 %.
Figure 6 Sample preparation procedure
The optimum asphalt percentage for each mixture is presented in Table 6. Th results
showed that the optimum asphalt content was increased with an increased rubber modifier
content. The higher optimum asphalt content of the mixtures is because of the thicker film of
the rubber crumb modified asphalt cement which coats the aggregates in the presence of the
rubber particles. The Marshall properties were also determined at the optimum asphalt
composition for the unmodified and modified mixtures as reported in Table 6.
Table 7 Optimum asphalt content of the asphalt mixtures
Marshal properties Mix type
Control mix 5% rubber
modified mix
10% rubber
modified mix
15% rubber
modified mix
Stability ,kN 10 10.2 10.3 10.7
Flow , mm 3.2 2.9 2.8 2.5
Bulk density ,
gm/cm3
2.38 2.38 2.372 2.366
Air void , % 3 3.4 3.8 4.1
Void filled
with asphalt , %
78 78 77 76
Void on mineral
aggregate , %
12.7 12.8 13.1 13.7
Optimum asphalt
content, %
4.8 4.9 5.1 5.4
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Figure 7 Result of Marshall test for the unmodified mixture
Figure 8 showed the Marshall stability values for the unmodified and modified mixtures
against their asphalt content. The results indicate that the stability values for different
mixtures followed a typical trend as a function of their asphalt content. The values were noted
to increase with an increasing asphalt content until a maximum value where stability tends to
decrease was reached. It also indicates that the Marshall stability for the modified mixtures
increased as the percentage of rubber content increased. The stability was also found to
increase as a function of the percentage of the added rubber crumb. The stability increased to
a certain value before starting to decrease; the highest stability valued achieved was 10.5 kN
from the mixture modified with 10 % CRM.
Figure 9 Marshall flow with asphalt
content for different mixtures
Figure 8 Marshall stability with different
asphalt content for mixtures
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Figure 9 presented the Marshall flow values with respect to the asphalt content for
different mixtures used. The flow values for the control and various modified mixtures were
found to increase with the asphalt content; this is commonly observed in asphalt mixtures
prepared with different asphalt contents. Figure 9 also showed a general increase in the
Marshall flow as the asphalt content increases. It was also noticed that the modified mixtures
gave lower Marshall flow values. The flow decreased as the rubber content was increased; the
maximum decrease was observed in mixtures modified with 15% CRM although this is still
within the standard specification limit (SCRB, 2003) [11].
5.2. Volumetric Properties of Asphalt Mixtures
The relationship between the air voids percentage in the total mixture and the asphalt content
in different mixtures is shown in Figure 10. The results showed the air void percentage to
decrease as the asphalt content was increased this is a common observation in asphalt
mixtures. The air void percentage of the modified mixture was also observed to be higher than
that of the control mixtures at different asphalt contents. The percentage of air voids filled
with asphalt against the asphalt content for different mixtures was shown in Figure 11. The
result indicates a similar VFA percentage trend as expected of asphalt mixtures where VFA
percentages tend to increase with the asphalt content. The %VFA of the modified mixtures
was also observed to be higher than those of the control mixture; the %VFA also increased
proportionally with the percentage rubber content.
The percentage of air voids in the mineral aggregate against the asphalt content for
different mixtures are presented in Figure 12. The results showed that the percentage of VMA
showed the typical trend expected from any asphalt mixture; the percentage VMA was found
to decrease as the asphalt content was increased; this decrease in percentage VMA persisted
until the minimum value was reached before the percentage VMA began to increase. It was
also observed that, for the same asphalt content, the percentage VMA of the modified
mixtures was higher than those of the control mixtures.
Figure 11 Air voids filled with asphalt
percent with asphalt content
Figure 10 Air void percent with asphalt
content for different mixtures
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Figure 12 Percentage air voids in the mineral aggregate with asphalt content
5.3. Ultrasonic Test
The ultrasonic device is used to measure the time it takes seismic wave pulses generated by a
built-in pulse generator to travel through a specimen. The generator transforms an electrical
pulse into mechanical vibration through a transducer. A receiver which is connected to an
internal clock records the arrival time of the seismic wave. This internal clock can
automatically measure and display the travel time of the waves. The resilience modulus of the
HMA specimens is determined from the travel time and specimens’ density. A major
advantage of this test is its non-destructiveness. Additionally, both laboratory-prepared
specimens and field cores can be used in this experiment.
The prepared specimens (as earlier described) were used for the ultrasonic tests. The
specimens’ elastic modulus was determined with the aid of an ultrasonic device made up of a
timing circuit and a pulse generator. The device was coupled with piezoelectric transmitting
and receiving transducers. A dominant energy frequency of 54 kHz was imparted to the
specimen. The time it takes a wave to travel through the specimen was digitally displayed by
the timing circuit. A maximum specimen-transducers contact was ensured by using special
detachable epoxy caps on the transducers. The transducer that receives the impulse is coupled
to an internal clock for sensing the propagating waves. The internal clock detects and displays
the travel time tv. This recorded time is used to determine the constrained modulus Mv
according to the specifications of ASTM (C 597 – 02).
Mv = ρVp² = ρ (L/tv) ² (1)
where: Mv is the constrained modulus, ρ is the density (g/mm3), Vρ is the velocity of the
compression wave (mm/ms), L is the average specimens’ length (mm), tv is the travel time
(ms).
In a simpler form,
Mv
(2)
where:
M is the specimens’ mass (g), d is the specimens’ average diameter (mm). Young´s
modulus Ev can be calculated as follows:
Ev = Mv [
(3)
The determination of the Poisson´s ratio υ is based on experience; it is generally taken to
be in the range of 0.3 - 0.4 for asphaltic materials [12].
The results of Young's modulus E at 0, 5, 10 and 15 % CRM percentage showed that E
increased with the CRM content of the asphalt. Figure 13 showed that E was gradually
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increasing with an increased rubber crumb content in the asphalt due to the increasing
elasticity factor, i.e, E values increased by 10, 5, and 4 % when 5, 10, and 15 % of CRM was
added, respectively.
Figure 13 Modulus of Elasticity E with rubber crumb content
The elasticity coefficient of the asphalt mixture can be increased by the addition of the
rubber crumbs to the original mixture since the elasticity factor is increased by increasing the
amount of CRM in the asphaltic mixture. This increase in elastic factor is because the CR
contains rubber materials.
6. CONCLUSIONS
Based on the experiments carried out in this research, it can be concluded that the physical
properties of rubberized asphalts which contain different percentages of rubber crumbs and
asphalt cement were influenced by the number of rubber crumbs in the mixture. The Marshall
stability of the modified mixtures increased with increasing rubber crumbs percentage. The
Marshall stability value of the control and modified mixtures at the optimum asphalt
percentage satisfied the engineering properties required by the SCRB/2003 specification for
asphalt mixtures used in the construction of surface course. The Marshall flow for the
modified mixtures showed an inverse relationship with the Marshall stability as the flow
values for various mixture was within the range 24 mm An increase in the percentage of
CRM in the modified mixtures decreased the bulk density of the mixtures. The air voids
percentage increased with increasing percentage of CRM; the control and all the modified
mixtures were within the specification range of 35%. The percentage of voids filled with
asphalt (VFA) inversely related with the percentage air void. The control and all the modified
mixtures showed a VFA percentage of 7085 percentage which satisfied the required
specification. The percentage of voids in the mineral aggregate (VMA) increased with an
increasing percentage of CRM content; the control and all the modified mixtures achieved the
minimum value of 12 %. The results of Modulus of Elasticity E was gradually increasing with
an increased rubber crumb content in the asphalt due to the increasing elasticity factor.
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